Method of combustion or gasification in a circulating fluidized bed

ABSTRACT

A material containing combustible components is burned or gasified in a circulating fluidized bed comprising a turbulence chamber, a solids separator connected to the upper area of the turbulence chamber, a return line leading from the solids separator to the turbulence chamber and a cooling device for the indirect cooling of solids arriving from the solids separator. The cooling device comprises several fluidized beds through which the solids pass one after the other. The first fluidized bed, into which the hot solids arriving from the solids separator are introduced first, is situated in a dechlorinating chamber. Fluidizing gas and at least one of the following dechlorinating additives: a) gaseous SO 2  or a material which contains sulfur and releases SO 2  in an oxidizing atmosphere, b) silicates and aluminium silicates, c) activated silicate, or d) other alkali-binding and HCl-releasing additives, are also introduced into the dechlorinating chamber in at least stoichiometric quantities so as to convert the alkali and metal chlorides contained in the arriving solids.

The invention relates to a method of combustion or gasification ofmaterial containing composite components in a circulating fluidized bedwhich has a turbulence chamber for the combustion or gasification, asolids separator connected with the upper region of the turbulencechamber, a recycle from the solids separator of the deposited solids tothe turbulence chamber and a cooling unit for the indirect cooling ofsolids which come from the solids separator, whereby the cooling unitincludes a plurality of fluidized beds through which the solids migrate,one after the other.

The method of this type is known from WO 97 46 829 A1. Here the coolingunit is preceded by an uncooled fluidized bed whose flue gas is feddirectly to the combustion. Because of this feature, the chlorideconcentration in the solids to be cooled are reduced. The chlorides areresponsible for aggressive corrosive attack on the cooling unit. Theknown blowing through of the fluidized bed is above all not suitable ininappropriate cases, of lowering the chloride concentration of thesolids to be cooled sufficiently to ensure the desired protectionagainst corrosion. Especially corrosive are, for example, HClencrustations on the cooling tubes.

The invention has the object of so treating the hot solids coming fromthe solids separator that their corrosivity in the cooling unitcompletely or substantially completely disappears. According to theinvention this is effected in the method described at the outset in thatthe first fluidized bed into which the hot solids coming from the solidsseparator is first conducted is a dechlorination chamber whereby in thisdechlorination temperature at a temperature of the solids in the rangeof 700 to 1100° C. and in the presence of a fluidizing gas and at leastone dechlorination additive which can be

a) a gaseous SO₂ or a sulfur-containing material that liberates SO₂ inan oxidizing atmosphere,

b) a silicate, laminate silicate,

c) activated silicate, or

d) other alkali additive compounds which liberate HCl, is added in atleast a stoichiometric quantity to convert the alkali and metalchlorides contained in the supplied solids.

As solid additives, various aluminum silicates, for example, kaolinitehave proved to be effective. Also suitable are activated silicates (forexample commercial ICA 5000), whereby the activation is achieved byboiling in sodium hydroxide. Economical is the use of waste substances,for example, chloride-free clarifier sludge or contaminated earth whichcontain these additives.

The reaction capabilities of the silicate, aluminum silicates, or theactivated silicates depends substantially upon the hydroxyl groups onthe silicon. These additives bind the alkali and metal in the hot solidsso that chlorine is liberated as HCl which is less corrosive than, forexample, alkali or metal chlorides. Usually these solid additives arefed in powder form into the fluidized beds whereby the mean particlesize d₅₀ lies approximately in the range of 50 to 500 μm. A gradualsupply of the solid additive to the feed line of the hot solids is alsopossible.

Gaseous SO₂ is above all suitable for reaction with alkali chlorides ormetal chlorides in a vapor form and thus to form sulfate and HCl in thepresence of molecular oxygen. The liberated HCl is driven off with thefluidizing gas from the dechlorination chamber. Sulfates are notcorrosive or are scarcely corrosive and can be disposed of with the ashdischarged from the process.

One can supply SO₂ in the gas space in a molar concentration of 0.25 toabout 6 times the concentration of the liberated HCl. The SO₂ can alsobe introduced as sulfur-containing materials which release SO₂ at thehigher temperatures in the dechlorination chamber or liberate SO₂ inoxidizing atmospheres.

The fluidized bed in the dechlorination chamber can operate with orwithout indirect cooling; usually this fluidized bed is maintained freefrom an indirect cooling. The remaining fluidized beds in the coolingunit contain heat exchangers provided by liquid gas or vapor coolants.By eliminating or at least reducing the corrosivity of the hot solids,one can maintain the temperature in the hottest fluidization bed highwhich, for example, can benefit the vapor superheating.

The material to be combusted or gasified can be of different types. Itcan thus be, for example, coal, lignite, biomass (e.g. wood or straw),solid and/or liquid wastes or clarifier sludge whereby, in addition,several of the aforementioned materials can be mixed.

BRIEF DESCRIPTION OF THE FIGURE

The variants of the process are detailed with the aid of the drawing.The drawing shows a flow diagram of the process.

The material to be combusted or to be gasified is supplied through theduct (1) to the turbulence chamber (2) which belongs to a circulatingfluidized bed. The upper region of the turbulence chamber (2) isconnected by the duct (3) to a solids separator (4) which can, forexample, be a cyclone. Partly dedusted gas is withdrawn via the duct,(5) and is supplied to a cooling and cleaning known per se and notillustrated. A part of the solids settling in the separator (6) arereturned to the turbulence chamber (2) by a recycle duct (6) via asiphon (7). The flow in the siphon is controlled by a fluidizing gasstream which is supplied in the duct (7 a).

The lower region of the turbulence chamber (2) has a grate (8) fromwhich the oxygen-containing fluidizing gas streams upwardly in thechamber. The fluidizing gas arrives by the duct (9) and initially passesthrough a distributing chamber (10) before it flows through the grate.Ash is removed via the outlet (11) from the chamber (2). The combustableor gasifiable material can be a variety of granular solids withcombustable components. In addition, liquid or pasty substances can beadded to it. The temperatures in the turbulence chamber (2) usually liein the range of 700 to 1100° C. and preferably 800 to 1050° C. With thefluidizing gas, a part of the solids is continuously passed via the duct(3) to the separator (4). The quantities of the solids which arerecycled through the recycle duct 6 into the turbulence chamber (2)usually amounts to at least 5 times the quantity of the solids, per hourwhich is on average found in the turbulence chamber (2).

A part of the hot solids settling in the separator (4), which has atemperature in the range of 700 to 1100° C. and usually 800 to 1050° C.are fed through the duct (14) of cooling unit (15). In the present case,the cooling apparatus (15) has a dechlorination chamber (16) and threecooling chambers (16 a), (16 b) and (16 c). The cooling chambers containheat exchangers (17) and (18) for indirect cooling of the solids whichare there provided as fluidized beds (21), (22) and (23). Between thebeds their weir-like chamber walls (21 a), (22 a) and (23 a). Fluidizinggas is supplied through ducts (21 b), (22 b) and (23 b). The fluidizinggas can be for example air.

The dechlorination chamber (16) has a feed duct (21 a) for fluidizinggas (e.g. air), this gas flowing through a grate (20 b) into a fluidizedbed (20) and then passing initially into the gas space (25) found abovethe fluidized bed (20). The gas space is also found above the otherfluid beds (21), (22) and (23). The gas discharge is effected via acollecting duct (26) which opens into the turbulence chamber 2 andthereby feeds the discharged gas of the cooled fluidized beds (21),(22), (23) through it. Alternatively, the dechlorination chamber (16)has its own gas discharge line (26 a) which is indicated in brokenlines.

To lower corrosivity of the hot solids past the duct (14) to a minimum,the dechlorination chamber (16) is provided with a feed duct (27) forsolid additives that feed duct (28) for gaseous additives. The gaseousadditives can be entirely or partly supplied also through the duct (28a). Solid additives are silicates, aluminum silicates and/or activatedsilicates or mixtures containing at least one of these additives. Asgaseous additives one utilizes gaseous SO₂ or other sulfur-containingmaterials which liberate SO₂ in oxidizing atmosphere. What is importantis that the content of alkali chlorides and metal chlorides which aresupplied with the hot solids through the line (14) are largely reducedby the additives. Preferably the solids which pass over the weir-likewall (21 a) of the dechlorination chamber (16) into the fluidized bed(21) have at most 20% of the alkali chloride and metal chloride contentwhich was present in the solids of line (14). The cooled solids whichpartly surrender their heat in the cooling device (15) in which as acoolant, for example, boiler feed water or steam can be used, asrecycled via the line (30) into the turbulence chamber (2). A part ofthe cooled solids can also be removed from the process although this hasnot been shown in the drawing.

EXAMPLE

In an apparatus corresponding to that of the drawing which, a part ofthe dechlorination chamber (16) has only two cooling chambers (16 a) and(16 b), a mixture is fed to the turbulence chamber (2) per hour and121,000 kg of granular coal and 41,000 kg of straw and burned at atemperature of 850° C. Through the line (14), one adds 16,200 kg/h ofash with a chlorine content of 0.002 weight % to the dechlorinationchamber (16) which has a base area of 1.5×0.8 and a height of 1.6 m. Theheight of the fluidized by (20) amounts to 1 m. The chamber (16) issupplied with a solids additive in the form of 23 kg/h of the activatedsilicate ICA 2000 the activated silicate ICA (manufacturer: ICA Chemie,A-3384 Gross Sierning) and has a gaseous additive with SO₂ through theducts (27) or (28) whereby in the gas space (25) a concentration of 30ppm SO₂ is established. From the cooling chamber (16 b) one withdrawsash cooled to 720° C. through the duct (30) and recycles it back intothe turbulence chamber (2). By the treatment in the dechlorinationchamber (16), the chlorine content in the solids is reduced to 10% ofthe starting content.

What is claimed is:
 1. A method of combusting or gasifying materialcontaining combustible components in a circulating fluidized bed whichhas a turbulence chamber for the combustion or gasification, a solidsseparator connected with the upper region of the turbulence chamber, arecycle from the solids separator of settled solids to the turbulencechamber and a cooling device for the indirect cooling of solids whichcome from the solids separator, in which the chlorine device includesseveral fluidized beds through which the solids migrate one after theother, characterized in that the first fluidized bed in which the hotsolids arrive initially form the solids separator, whereby withfluidizing gas at temperatures in the range of 700 to 1100° C. and thepresence of at least one dechlorination additive in the form of a)gaseous SO₂ or sulfur-containing material which liberates SO₂ inoxidizing atmosphere, b) silicates and aluminum silicates, c) activatedsilicates, or d) other alkali binding and HCl liberating additives whichare supplied in at least stoichiometric quantity, the alkali quantitiesand metal chlorides of the solids supplied are reactivated.
 2. Themethod according to claim 1 characterized in that the fluidized bed inthe dechlorination chamber is free from an indirect cooling.
 3. Themethod according to claim 1 characterized in that the silicates,aluminum silicates and/or activated silicates are added to the fluidizedbed of the dechlorination chamber and that gaseous SO₂ or othersulfur-containing materials which liberate SO₂ in oxidizing atmospheresare supplied to the gas space above the fluidized bed in thedechlorination chamber.
 4. The method according to claim 1 characterizedin that the material to be combusted or gasified contains solid and/orliquid wastes.